The invention relates to a device and method for variably braking a running yarn in a weaving machine.
A controlled yarn brake expediently is employed on a rapier weaving machine or a projectile weaving machine to produce a predetermined yarn tension profile in the yarn inserted into the weaving shed, which yarn tension profile is important for an optimum insertion cycle. Said yarn tension profile varies during the insertion cycle. In a rapier weaving machine it is e.g. of advantage to first brake stronger at the start of the insertion, to reduce or to nullify the braking effect during the subsequent acceleration phase of the bringer gripper, to again brake stronger during the transition phase from the bringer gripper to the taker gripper, then to reduce or nullify the braking effect at least for the acceleration phase of the taker gripper, and finally to brake again stronger in the end phase of the insertion until the inserted yarn is released by the taker gripper.
A controlled yarn brake known from EP-A-0 524 429 either does not brake at all or brakes with a single value of the braking force as adjusted at an adjustment device. The braking force can be selected manually at the adjustment device, however, due to the short time of an insertion cycle a further variation of that braking force is impossible during the same cycle. Since the braking force is identical for all operation phases it has to be a compromise such that the braking effect might be too weak for one operation phase, but may be too strong for another one. It is important to brake the yarn during differing operation phases of an insertion cycle with a varying braking effect.
It is known from practice to vary the braking effect of a controlled yarn brake by an actuator, e.g. by means of a rapidly responding stepper motor, in order to fulfill the requirement of differing braking effects for different operation phases of an insertion cycle. Due to the considerably short duration of an insertion cycle and the function depending inertia of the actuator, a relatively long period of time may elapse before the braking effect actually is nullified or activated or the respective adjusted force is fully active. Frequently, an undesirable residual braking effect remains even after nullifying the braking effect or the desired braking effect is not reached at the right point in time after actuating the braking effect. Generally, either too much braking power is imparted onto the yarn, or the actual braking effect as achieved at predetermined points in time or predetermined angle values of the rotation of the main shaft of the weaving machine is too weak.
It is an object of the invention to provide a device which is structurally simple and allows, during each insertion cycle, an optimally timed yarn tension profile to be achieved, and to provide a method by which an optimum yarn tension profile can be achieved for each insertion cycle.
This object can be achieved by providing a device for variably braking the running yarn which has two opposed braking elements, a first actuator for temporarily nullifying the braking effect, a control device connected to the first actuator, and an adjustment device for adjusting the force of the braking elements and actuable by a second actuator when the braking effect is nullified by the first actuator, wherein the second actuator is also connected with a control device.
The second actuator in the device varies the force decisive for the braking effect exclusively when the braking effect is nullified. Hence, the required braking effect is achieved exactly at the point in time or at the angle position of the main shaft of the weaving machine at which then the first actuator rapidly activates the braking effect. Since the time period between operation phases with activated braking effects is used to vary the force, and since the first actuator is capable of activating or nullifying the braking effect extremely rapidly, an optimum tension profile of the yarn can be achieved. This results from the fact that the first and second actuators divide the two tasks of activating or nullifying the braking effect and varying the force.
In view of the method it is of importance to use only operation phases to vary the force, during which operation phases the braking effect is completely nullified. This avoids a disadvantageous or delayed variation of the force during a subsequent operation phase for which the full braking effect is required.
Expediently, a second actuator of an electric, an electromagnetic, piezo-electric, electro-mechanic, pneumatic or hydraulic kind is used. Said actuator uses the time periods of the nullified braking effect to vary the force. To the contrary, the first actuator is only responsible for rapidly activating and nullifying (switching on and off only) the braking effect.
In preferred embodiment, both actuators are connected to a common control device. Said control device is adapted to control the second actuator exclusively when the first actuator has already nullified the braking effect.
It is of a particular advantage to use the second actuator in a so-called window-lamella-yarn brake to vary the contact force of the spring lamella co-operating with the counterstay bolt. The bolt has the window which is used to nullify the braking effect. The second actuator can be controlled in a simple way, because the predetermined rotational positions of the counterstay bolt or the first actuator are known precisely when it nullifies or activates the braking effect. A counterstay bolt can have only one circumferential window or even may have several circumferential windows. Said counterstay bolt then co-acts with the first actuator which either rotates back and forth in consecutive steps with one sense of rotation only.
Alternatively, the second actuator can be used to vary the force in controlled band brakes or disk brakes when the braking effect is nullified. A prerequisite is, however, a first actuator being fast enough to activate and nullify the braking effect rapidly.
The spring lamella of the window-lamella-yarn brake may be provided at a rotationally supported holder, the rotational position of which determines (with the inherent force of the spring lamella) the braking force. The second actuator forms a rotational drive for the holder. In this case the second actuator ought to be designed such that it automatically maintains the rotational position corresponding with predetermined forces.
In the control device, different rotational positions for the holder may be stored such that they can be established selectively via the second actuator. Said differing rotational positions represent the different forces of the braking effects.
Alternatively, the control device may contain a logical actuation inhibition system hindering the second actuator to vary the force until the braking effect has been nullified by the first actuator.
In another alternative embodiment, the second actuator may be adjusted back and forth between at least two position representing predetermined values of or a predetermined ratio between the values of the force. Nevertheless, said predetermined values of or the ratio between said values may be varied at the control side.
Expediently, the adjusting device for the force additionally is provided with a manual adjuster. In this case, the force also can be varied manually as in case of conventional window-spring lamella-yarn brakes, but even during an insertion cycle.
Expediently, the second actuator is a switching magnet, a rotary solenoid, a permanent magnet motor, a stepper motor, a LAT electro-motor (low angle turn electro-motor) or a hydraulic or pneumatic motor. Such types of actuators are sufficiently reliable as the second actuator may have a slower response behaviour than the first actuator, because it can use the nullified braking effect time periods during an insertion cycle to vary the force. The force variation, in practice, is typically relatively small.
Provided that the control device comprises a microprocessor, the force determining the braking effect even can be varied steplessly via the second actuator.